CONDITIONS OF EXISTENCE 



59 



IV. Lake-Pond Communities (see Figs. 7 and 8) (Shelf ord). 



1. Pelagic sub-formations 



2. Terrigenous bottom formations 



3. Vegetation formations 



a. Submerged vegetation associations 



b. Emerging vegetation associations 



4. Temporary pond formations (Shelford) 



Conditions of existence in fresh water at any given point are 

 changing in a definite direction. This change involves every item 

 of the environment which has been enu- 

 merated on the preceding pages. Streams 

 wear down their beds, wear their valleys 

 wider, reduce the speed of their current, 

 grind their coarse bottom materials into 

 the finest silt. The waves of lakes cut 

 away the shores, grind up the rocks they 

 break off in this process, and deposit the 

 silt thus produced in the bottom. Streams 

 lower the outlets of lakes and carry detri- 

 tus into them. 



Ponds and small lakes support vegeta- 

 tion which decays, filling their bottoms 

 with putrescible material which is gradu- 

 ally transformed to humus with a lowering 

 of oxygen and the development of poison- 

 ous decomposition products. The ponds 

 and lakes are thus filled as well as drained the region of emerging vegetation. 



Crosses indicate the region of sub- 



and all become swamp and finally dry land, merged vegetation, stippling indi- 



cates the region of deep water or the 



FIG. 8. 



Diagrammatic representation of a 

 lake in surface view. Vertical dashes 

 mark the region of erosion and sandy 

 bottom. Horizontal dashes indicate 



Streams gradually erode their way down 

 to sea level and become meandering base 

 level streams with fine silt bottom, sluggish (On s inaL) 

 current and an abundance of vegetation. The base level streams 

 and dry land are the ultimate fates of all bodies of fresh water. 

 With the changes enumerated, there is always almost complete 

 change of animal and plant life. The physiological requirements 

 of the life of the first stages of the process are entirely different 

 from those of the last. 



